How would you make a program that uploads files to sites for example?

How would someone make a program that uploads files to say, Mediafire or a booru, other than an API?

Someone asked me once to make a thing that dumps all the images in a selected folder to a booru, and I got the part with the folder of images down pretty easily, but I couldn't figure out a way to get those images to be uploaded to the site, as it didn't have an API.